1. Field of the Invention
This invention relates generally to apparatus for controlling the magnitude of alternating electrical current and more particularly, but not by way of limitation, to apparatus for balancing the magnitudes of the phase currents of a three-phase power system. 2. Description of the Prior Art (Prior Art Statement)
The following statement is intended to be a prior art statement in compliance with the guidance and requirements of 37 C.F.R. .sctn..sctn.1.56, 1.97 and 1.98 and of .gtorsim.6.09 of the Manual of Patent Examining Procedure.
A number of previously issued patents have disclosed several apparatus for equalizing or balancing the magnitudes of currents or voltages in multi-phase power systems. Two of these patents known to Applicant are believed to be the most pertinent. The first is the patent issued to Rau, U.S. Pat. No. 2,808,519, which discloses a voltage equalizer system wherein two phases of a three-phase system are used to control the third phase. This equalization is accomplished by rectifying two of the phases to provide direct currents to control an adjustable reactance device in the third phase which in turn controls the current of the third phase. From the nature of the elements described in the patent, the third phase is maintained at a magnitude which is the average of the magnitudes of the two controlling phases. The Rau patent further indicates that this type of system is used to compensate for an imbalance created by a single phase load which is connected across the two phases which are used to control the third phase.
Second, the patent issued to Schaefer, U.S. Pat. No. 3,171,967, discloses a device for balancing the magnitudes of all phase currents in a multi-phase rectifier circuit. This device uses each phase current to control a respective current in another phase. This control is accomplished by using adjustable reactor cores which are interconnected with the phase lines.
Additional patents and disclosures which are known to Applicant and believed to be of interest include:
Dreyus, U.S. Pat. No. 3,193,605;
Thompson et al., U.S. Pat. No. 3,991,359
Blackmond, U.S. Pat. No. 3,676,766
Wickenhagen, U.S. Pat. No. 3,292,076
Balteau, U.S. Pat. No. 3,075,139
Morgan et al., U.S. Pat. No. 3,045,126
Hage, U.S. Pat. No. 2,798,196
ESCO, Negative Sequence Compensator
As shown by the above-mentioned disclosures, there is a need for a compensating apparatus to balance the magnitudes of every phase current in a multi-phase power system. There is also the need for such a device to automatically balance the phase currents without having mechanical or other types of adjustments made to it. Furthermore, there is the need for a compensator which satisfies the above two needs and yet is efficient in size, weight and cost.
The previous disclosures, however, do not describe a compensator which meets these needs while automatically balancing every phase regardless of the cause of unbalance. Additionally, the devices of the prior disclosures have disadvantages which the present invention does not have.
One disadvantage of the Rau patent is that it fails to disclose a device for compensating more than a single phase. Furthermore, the compensation resulting from the Rau system does not balance the phase currents to the degree that the present invention does because due to the nature of the reactance adding device described therein, the magnitude of the compensated phase is only the average magnitude of the two controlling phases. Although the average of two equal phases would result in the third phase also being equal, generally no two phases of a three-phase power system are identical. Additionally, because only one phase contains the reactance adding device, the other two phases remain uncompensated. Finally, the device disclosed in the Rau patent fails to compensate any cause of unbalance unless the two controlling phases are lower than the uncompensated third phase.
A disadvantage of the Schaefer patent is that it fails to disclose a device which automatically balances all phases because the device requires adjustment to be made to the control circuit. Furthermore, the Schaefer patent fails to describe a compensator which balances without regard to the timing differences between the phases.
The other cited patents also have disadvantages because they fail to disclose the relatively simple, individual phase controlling, automatic current balancing compensator of the present invention.
In light of these disadvantages I believe that no previously disclosed device which is known to me indicates, either singly or in combination, the present invention.